Polylactate (PLA) is a typical biodegradable polymer derived from lactate that is highly applicable commercially and biomedically. Although preparation of PLA presently involves polymerization of lactate produced by fermenting microorganisms, only PLA with a low molecular weight of about 1000 to 5000 daltons is obtained by direct polymerization of lactate. In order to synthesize PLA with a molecular weight of 100,000 daltons or higher, PLA with a low molecular weight obtained by direct polymerization of lactate may be polymerized using a chain coupling agent. In this method, however, the entire process becomes complicated due to addition of an organic solvent or the chain coupling agent, which are not easy to remove. A presently commercially available process of preparing high-molecular weight PLA may include converting lactate into lactide and synthesizing PLA using ring-opening polycondensation of lactide rings.
When PLA is synthesized by chemical synthesis of lactate, a PLA homopolymer is easily obtained, but a PLA copolymer composed of various types of monomers is difficult to synthesize and commercially unavailable.
Meanwhile, polyhydroxyalkanoate (PHA) is polyester stored by microorganisms as energy or a carbon source when there are excessive carbon sources and a lack of other nutrients such as phosphorus (P), nitrogen (N), magnesium (Mg), and oxygen (O), etc. Since PHA has similar physical properties to conventional synthetic polymers from petroleum and exhibits complete biodegradability, it is being recognized as a substitute for conventional synthetic plastics.
In order to produce PHAs using microorganisms, enzymes converting microbial metabolic products into a PHA monomer and a PHA synthase for synthesizing a PHA polymer using the PHA monomer are needed. When synthesizing PLA and PLA copolymer using microorganisms, the same system is required, and enzymes for providing lactyl-CoA are needed in addition to an enzyme for providing hydroxyacyl-CoA, which is an original substrate of a PHA synthase.
Furthermore, it is very important to adopt a low-cost substrate in order to economically produce biodegradable polymers. In particular, it is necessary to develop a technique of producing a PLA or PLA copolymer using sucrose as a low-cost substrate.